Abrasives are commonly used to grind, sand, and polish materials such as wood, metal, and plastic. Grinding, sanding, and polishing may be generically be referred to as abrading, because each process involves the removal of material, either on a macroscopic or a microscopic scale, due to abrasion between contacting surfaces in relative motion. One method of abrading a workpiece is to affix an abrasive sheet member to a back-up pad, and to rotate the back-up pad while urging the abrasive against a workpiece. In many applications, the abrasive member and the back-up pad are circular, and although the abrasive member will be referred to as an abrasive disc hereinafter, other configurations are also known. For example, U.S. Pat. No. 4,920,702 (Kloss et al.) discloses a portable grinder having, in one embodiment, a generally triangular back-up pad and abrasive disc that are vibrated rather than rotated.
The abrasive disc may be affixed to the back-up pad in a number of different ways. For example, the use of a pressure sensitive adhesive (see U.S. Pat. No. 3,849,949 (Steinhauser et al.)), interengaging fastener members, such as a multiplicity of hook portions on the back-up pad and a multiplicity of loop portions on the abrasive disc (see U.S. Pat. No. 4,609,581 (Ott)), and cooperating male and female fastener members, are known in the art. The back-up pad may be constructed of various materials (e.g. rubber) depending on the desired stiffness of the back-up pad. The diameter of the abrasive disc is typically larger than the diameter of the back-up pad (for example, a 127 mm disc and a 124 mm pad), in order to permit abrading near the periphery of the abrasive disc without damage to the back-up pad. Because an even finish may be important, it is also desirable to center the abrasive disc on the back-up pad so that the amount by which the disc overlaps the back-up pad is relatively constant along the periphery of the disc.
The back-up pad is attached to an abrading apparatus, which includes a rotating output shaft that is powered by an abrading head. These abrading apparatuses, alternatively referred to as grinders herein, may be either electrically or pneumatically powered, and typically rotate the back-up pad at speeds between 3,000 and 20,000 revolutions per minute. The abrading apparatus may be one of several types. For example, a rotary grinder simply rotates the back-up pad and attached abrasive disc about a fixed axis. This may cause the abrasive face of the disc to abrade deeper scratches into the surface of the workpiece, because the abrasive face follows the same path during each rotation of the disc. This regular rotary motion can result in deeper, coarser cutting, which may be desirable for some applications, but not for others.
The particular type of grinder with which the present invention is concerned is the random orbital grinder. This type of grinder combines a rotary and orbital motion which results in a random motion of the back-up pad with respect to the abrading head. Such a motion is desirable because a random motion of the abrasive decreases the likelihood that a regular pattern of deeper scratches will be cut into the surface of the workpiece. As a result, a finer finish may be obtained on the surface of the workpiece. The random motion may be generated by a structure akin to that shown in FIG. 1. Abrading apparatus 10 includes abrading head 11, which carries drive shaft 12, which is rotated on journal bearing 16 about axis A--A by power source 18, which may be, for example, electric or pneumatic. It should be understood that the terms "rotate" and "revolve" are not intended to be synonymous for purposes of the present invention. Rotation herein refers to the angular movement of a body about the central axis of the object. Revolution herein refers to the angular movement of a body about an axis spaced from the central axis of the object. In the context of the illustrated abrading apparatus 10, housing 20 rotates with drive shaft 12 about axis A--A, and houses second shaft 22, which rotates with journal bearings 24 and 26 about axis B--B. Axis B--B is parallel to but spaced from axis A--A, and therefore second shaft 22 tends to revolve around axis A--A, while rotating about axis B--B.
Second shaft 22 carries part 30, including counterweight 32, which counterweight assists in effecting revolution of part 30 about axis A--A. It should be noted that the speed of rotation of part 30 about axis B--B may depend on parameters such as the amount of force applied to the grinder, the material composition and topography of the workpiece, and the abrasive that is used. For example, under very light pressure, part 30 may rotate relatively quickly whereas under a very high load, part 30 may rotate relatively slowly, or not at all, although it would probably continue to revolve about axis A--A. Back-up pad 34 is connected to part 30, and has abrasive disc 36 releasably mounted thereon. The general operating principles of a random, orbital grinder are further described in a variety of sources known in the art, including U.S. Pat. No. 4,660,329 (Hutchins), the contents of which are hereby incorporated by reference.
The combined effect of the revolution and rotation of part 30, and therefore of back-up pad 34 and abrasive disc 36, produces a random, orbital sanding motion. The random action of the abrasive results in a finish that is finer than would result from a rotary grinder, because the path of the abrasive disc is random with respect to the workpiece, which helps to prevent the abrasive disc from abrading deeper scratches during each rotation.
Although manually abrading a workpiece is advantageous under certain circumstances, the cost of manual abrading may be excessive for large-scale manufacturing operations. When a continuous series of identical workpieces are to be abraded in an identical manner, manufacturers have found it useful to implement robots to abrade each successive workpiece identically. Robotic apparatuses can be programmed to follow a predetermined sequence of commands that position an attached grinder at a specific location to abrade the workpiece, and therefore to complete the abrading process with a minimum of human intervention.
In order to abrade each workpiece effectively, a worn abrasive disc must be replaced periodically with a new abrasive disc at the end of an abrading cycle. As used herein, an abrading cycle begins when the disc is applied to the back-up pad and ends when the disc is to be removed. In most cases, the abrading cycle corresponds to the useful life of the disk, but the abrading cycle may be shorter if, for example, a finer or coarser abrasive disc is desired. For simplicity, the abrasive disk that is to be removed at the end of the abrading cycle will be referred to as the worn abrasive disc. If a human operator must replace the worn abrasive discs at regular intervals, which may be relatively often depending on the abrasive and the material from which the workpiece is constructed, the benefit of having a robotic apparatus replace a human operator is diminished. Thus it is desirable to provide an integrated, automated system for removing and replacing a worn abrasive disc.
To program an automated apparatus to remove a worn abrasive disc, the position of the disc at the completion of the abrading cycle must first be ascertained. In the case of a rotary grinder, determining the location of the disc is relatively simple, because the position of the back-up pad and disc does not change with respect to the position of the abrading head. However, in the case of a random, orbital grinder, the back-up pad and attached abrasive disc will likely be at a different location at the end of each cycle, because the back-up pad does not follow a designated path--its action is random within a measurable range of motion. For example, a possible range of motion of a back-up pad with respect to the abrading head is shown in FIG. 3 by distance "d". It is therefore more difficult to consistently locate the back-up pad and disc with respect to the abrading head of a random, orbital grinder than of a rotary grinder.
In order to grasp the worn disc that is attached to the back-up pad, the robotic apparatus must be programmed to present the disc at a constant, specified location at the end of each abrading cycle for convenient engagement and removal by a grasping device. However, the device typically grasps only the outermost edge of the disc, and thus even minor variations in the location of the disc may prevent an automated grasping device from grasping the disc. As shown in FIG. 3, the position of the back-up pad (34 and 34') at the end of an abrading cycle may vary by a distance "d" due to the cooperative movement of the illustrated components. Because the abrasive disc overlaps the back-up pad by only a small amount, an automated grasping device that is programmed to grasp an edge of an abrasive disc at a certain location may, depending on the point at which the back-up pad stops, be unable to grasp the abrasive disc. Thus the automated grasping means may be unable to locate the disc for removal unless the back-up pad is uniformly positioned at a specific point in its range of motion. It is the necessity for precisely locating the back-up pad within its range of motion with which the present invention is particularly concerned.
Even if the worn abrasive disc is somehow removed from the back-up pad at the end of the abrading cycle, other problems may result from the inability to precisely locate the back-up pad and abrasive disc at the end of each abrading cycle. If the location of the back-up pad is not precisely determined at the end of the abrading cycle, the new abrasive disc may not be centered on the back-up pad. The improper alignment of the disc with respect to the pad could cause the edges of the abrasive disc to tear, to abrade unevenly, or even to detrimentally affect the grinder bearings due to the imbalance. Furthermore, an incorrectly positioned disc could fly off during grinding, which could potentially injure a person or property. An abrasive disc that is not centered on the back-up pad could expose a portion of the edge of the pad, which can damage both the back-up pad and the workpiece. Finally, it may be difficult to abrade edges or contoured surfaces with an abrasive disc that is not centered, because the some portions of the periphery of the abrasive disc may abrade more material from the workpiece than other portions that may not overlap the back-up pad by as great an amount.
It is therefore desirable to provide an automated random, orbital grinding system including means for locating and removing an abrasive disc from the back-up pad thereof, as well as a method for positioning the back-up pad of a random, orbital grinder at a known location within its range of motion at the end of an abrading cycle.